Sealants and caulks are used for a variety of purposes, such as to fill in cracks, crevices or gaps in structural units and to fill spaces between neighboring panels or building units. The caulks and sealants function to protect the sealed surfaces against weather conditions and, accordingly, they must be capable of withstanding wide temperature, moisture, and sunlight exposure conditions in order to be successfully employed. These materials should be capable of absorbing shear, compression, and extension stresses exerted thereon caused by shifting movement of one structural unit relative to its neighboring unit. Ideally, the caulk or sealant should be capable of receiving paint and provide an acceptable adherence location therefor.
Various sealant and caulk types are presently in use. For example, polyurethane sealants provide versatile performance with excellent elongation and recovery and good tear strength and abrasion resistance. Most urethane sealant systems are based on reaction of polyhydroxy compounds with polyisocyanates to form polyurethane or urethane-urea elastomers.
In two-part urethane sealant compositions, an isocyanate source, typically a so-called urethane "prepolymer" is provided as one component to be mixed with a second component containing available hydroxyl functionality. Curing catalysts, such as methylene dianiline, fillers, and pigments may also be included within one or both of the formulation parts.
One-part moisture curing urethane sealant compositions are known in which, typically, an isocyanate prepolymer is mixed with fillers and pigments. The prepolymers cure via moisture absorption upon usage but curing catalysts, such as 1,2,4-trimethylpiperazine, can be used to speed the cure, but commonly require sacrifice in component storage life.
Silicone rubber sealants and caulks are also used and provide excellent heat resistance, low temperature flexibility and durability characteristics. Silicone sealants can be provided in either one-part or two-part formulations with the two-part mode usually comprising an hydroxyl-terminated polysiloxane and a cross-linking agent, such as ethyl orthosilicate packaged together as one of the parts. An organometallic activator or catalyst, such as dibutyl tin dilaurate is provided as the other part, with mixing of the two parts needed for curing.
In one-part silicone approaches, methyl substituted polysiloxanes having hydrolyzable end groups are commonly employed. When the component is subjected to atmospheric moisture upon usage, the hydrolyzable end groups are volatilized and the silicone effects cure through the formation of transient Si--OH bonds which condense to more stable Si--O--Si structure. The volatile hydrolyzable groups increase VOC emissions and, in some cases, can increase corrosion of the sealed structural units and create malodorous conditions.
As used herein, the terms "sealant" or "caulk" will be used interchangeably to denote the instant compounds that are used to fill and seal a joint. A sealant is bonded to the sides of a joint such that watertightness is retained even upon minor movement of the joint.
In contrast, a sealer is a liquid that is applied to a relatively porous surface to seal it and improve the adherence of the finish material or to waterproof the surface.
Accordingly, it is an object of the present invention to provide a storage stable one-part sealant composition in which the beneficial attributes of one-component urethane and silicone sealants are combined in a moisture curing sealant composition.
It is another object to provide such a composition that, unlike traditional one-part silicone formulations, does not contribute significant VOC emission upon moisture cure.
It is a more specific object to provide a silicone-urethane one-component sealant/caulk composition that exhibits enhanced toughness and anti-yellowing characteristics.